This invention relates to a method for electrical discharge machining wherein the workpiece surface is etched by electrical discharge machining to a predetermined surface roughness for providing the satinized indented surface on the workpiece. Hereafter, such machining method and the work in the form of a roll will be abbreviated occasionally to electrical discharge dull machining and the work, respectively.
Heretofore, in providing a satin finish to a roll used for rolling a steel skelp, especially to a cold working roll, hard metal particles known as shots or grits are projected to a ground roll surface for providing indented marks on the roll surface. In recent years, this type of machining is tentatively performed by an electrical discharge machining. The electrical discharge machining as used in such method consists essentially in providing an insulating liquid such as kerosene in the narrow discharge gap between the electrode and the work surface and a pulsed voltage is applied periodically across the electrode and the work to cause an electrical discharge for etching or machining the work surface thereby. Such discharge is caused to occur repeatedly on the roll surface while the roll is rotated about its axis and moved longitudinally along the rotary axis of the roll for providing a satin finish on the roll surface consisting of spirally extending continuous discharge marks. This is the method for providing a uniform satin finish on the roll surface by utilizing an electrical discharge machining. The satinized roll surface thus obtained has many advantages over that obtained through projection of metal particles in that the indents are more accentuated and trim in shape, while not being influenced by the degree of hardness or the manufacture process of the roll, and in that the metal structure at or near the roll surface may be improved in toughness due to electrical discharge and thus may be most convenient for a roll used in the roll working.
According to the conventional practice for such electrical discharge dull machining operation, multihead and multi-divided electrodes are used with a view to reducing the working time. In this case, on account of certain characteristics of the electrical discharge machining operation, the working time is increased markedly in the region of lower surface roughness of 6 to 10 .mu.RZ as compared to the conventional machining methods using metallic particles such as shots.